Norwalk is a small RNA virus about 400 _ in diameter that causes gastroenteritis. This virus can be obtained in large quantity from a baculovirus-expressed system. Large crystals of Norwalk virus have been obtained and found to diffract beyond 3 _ by Dr. Prasad in collaboration with Dr. Michael Rossmann. They are interested in using the highest possible structure from 400 keV cryomicroscopy to help phasing the X-ray diffraction intensity. They are trying to solve its structure without requiring a heavy atom derivative.